Those [National Union 6F8G] are apparently a little hard to find. Would you have any suggestions on where to find some? I've checked most of the common sources as well as a general Google search. Thanks.

I've own a few Raytheon 6F8G and those are the only ones I've heard. I wonder how they compare to the NU tubes.

So I was listenin' on my WA6-SE tonight and suddenly I get some very loud POPs in my headphones (left channel only). My wife was also in the room and said it sounded like a light blub went out. I thought it was a one time deal and about 20 minuets later I get the popping sound but now instead of just one I get a series of pops (again on the left side). I immediately unplugged my headphones, but I could still hear the pops from the amp itself (but I didn't notice any lighting or sparks or anything like that).

I turned off the amp and swapped my driver tubes and powered back up and I immediately got a few more pops (without the headphones pluged in, so not sure which side it was), but now it has settled down.

Should I be concerned? These are NOS 6EM7s and NOS 598s with maybe 10 hours or so on them.

Does it pops only during warm up or continues thru out? I would not try again if it continues, it may even damage the amp.

It started happening maybe 40 minutes into the listening when the problem showed up. I swapped the tubes from left to right. I didn't hear nay pops but if the popping switches channels I'll know and get new tubes.

It started happening maybe 40 minutes into the listening when the problem showed up. I swapped the tubes from left to right. I didn't hear nay pops but if the popping switches channels I'll know and get new tubes.

I posted this in the Lyr thread but also wanted to share my impressions with fellow Woo Audio owners. All listening was done with RFT EZ81 rectifiers and Tung Sol 7236 power tubes, with both the HD600's and HD650's.

Amperex Orange Globe 6DJ8 - A Frame

These are both from 1973 and made in the Heerlen plant in Holland.

I consider this to be a great tube for the HD600's, as they have a bit more lower end punch and a super smooth and lush midrange. Yet, they still have a great airy top end. They are the Sennheiser of tubes! With the HD650's, they sound a bit too warm for my liking. They are still incredibly detailed and provide good separation, however I wouldn't consider these to be perfect all-around tubes and prefer listening to them with my HD600's.

I find myself listening to these tubes when I just want to space out and listen to music. They're so far from being fatiguing that I could spend days listening to these.

RTC E188CC Green Label - Large "O" Getter

These were loaned to me by an awesome Head-Fi'er (sceleratus). These babies are from 1965, and from what I can tell, were made in a Phillips-owned factory in Suresnes, France. sceleratus may be able to confirm that.

To my ears, these aren't quite as warm and lush as the Amperex's. However, the first thing I noticed was the incredible detail and separation between each instrument.

#36, a track from Disc 2 of Dave Matthews Band "Live at Red Rocks" album starts out with a really good drum solo. It was super easy to pick out the location of each strike, yet they are accurate enough as to not drown out the delicate high hats while Carter Beauford is wailing away on the kick drum. Hands down the best I've ever heard this track reproduced.

While they sound great with the HD600's, I think they're better suited to the HD650's. Also, unlike the Amperex Orange Globes, it took some time for me to really appreciate what they have to offer. These tubes are truly a treat, and probably one of the best I've ever heard.

Valvo E88CC - Large "O" Getter

These are from 1960 and 1964, and were also made in the Heerlen plant.

These tubes seem to combine my favorite things from both the Amperex 6DJ8's and RTC E188CC's. They are incredibly airy and delicate, yet have a touch more body and a warmer midrange than the RTC E188CC's, slightly reminiscent of the Amperex 6DJ8's. I've always preferred the HD650's over the HD600's, however with these tubes driving my HD600's I honestly can't find a single flaw and find them to be a perfect match, just like the RTC's and the HD650's. For those that enjoy the Sennheiser sound signature, these tubes are a must have. After listening to the RTC's, I was bummed that I'd eventually have to return them. The Valvo's changed that

And of course the trio, from left to right...Amperex 6DJ8 -> RTC E188CC -> Valvo E88CC

If other tubes are fine, your amp is fine. Did yo have a tube tester? Where did you get the tube from?

No tester, no that hardcore. lol.

got the tubes from http://www.vacuumtubesinc.com/ the owner is a GREAT guy and I've done lots of business with him before including purchasing a few 596s. (Yes SO, I meant 596, although I love my Senn 598s too.)